1. Field of the Invention
The present invention relates to a flow control apparatus for use with a power steering device in which an operating fluid discharged from a pump is delivered to a power steering device through a restricted passage and an excess flow is returned to the suction side of the pump through a bypass passage.
2. Description of the Prior Art
In a conventional flow control apparatus for a power steering device, a relief valve for setting the maximum load pressure is contained in the flow adjusting spool valve, which controls the flow area of the bypass passage in response to the pressure difference across the restricted passage. With this arrangement, the flow of the operating fluid delivered to the power steering device is maintained at a predetermined value. In addition to that, the load pressure is adjusted by the relief valve not to exceed a predetermined allowable maximum pressure.
The prior flow control apparatus is able to prevent the load pressure from exceeding the maximum load pressure at the usual operating temperature, because the relief valve operates very quickly in response to an increase of the load pressure. However, the prior flow control apparatus is not able to prevent the load pressure from exceeding the maximum load pressure during starting periods under low temperature conditions, at which the temperature of the operating fluid also becomes low. Namely, since the viscosity of the operating fluid becomes high under such conditions and the load pressure is transmitted to the relief valve though a damping orifice and a connecting passage, the load pressure, which increases in response to actuations of the power steering device, is transmitted to the relief valve with a large time delay, thereby the load pressure of the power steering device sometimes exceeding the allowable maximum load pressure.